Heavy vehicle hissing?

Recently while travelling on a bus down a fairly steep descent, I became very aware of the regular hissing of, what I'm assuming were, the brakes - it would come at fairly regular intervals, a voluble burst of hiss, I think I might've been more aware of it because of where I was sitting on the bus. I've heard it from many other heavy vehicles, but what is it exactly? exhaust braking?

The hissing sound is because instead of fluid like a car would use, the truck uses compressed air to actuate the brake pads. They use very large diaphrams because the pressures are lower.

When the truck is hooked up a trailer or should it be disconnected all that comes out of the hose is air. With brake fluid in a car, air in the system is very bad because of the high pressures and compressibility of air. One air bubble in a fluid brake system could reduce the pressures from 1000psi to 10psi because of this, meaning you'd effectively have almost no brakes. Removing the air is a process normall called "bleeding the brakes" and this also removes the old fluid. The fluid is hydroscopic, meaning it absorbs water and if the fluid with water would get too warm a steam bubble would form with the same results as air bubbles.

The trucks are also designed with spring backups, so if you would cut the air hose or the system would loose all pressure for some other reason all the brakes come on at full power on the truck to prevent a runaway truck without brakes.

I'm not that sure about this (think i heard it somewhere) but i thought the air pressure is what held the brake pads off the brake discs. And the hissing was the release of this pressure as the brakes are being applied. Might have just dream't that though.

The air brake systems also actuate a discharge valve located on the bottom of a liquid accumulator. This is designed to remove any accumulated moisture from the system. This produces a periodic [every few minutes?], quick burst of air that can be heard.

Not quite. There are 2 separate diaphrams for each wheel on a truck with air brakes. They are combined into one canister so it looks like one. The canister contains a spring that holds the brake on if there is no air pressure at all due to broken lines and etc. This is known as the parking brake and when a truck is parked a valve is flipped that bleeds the air from that part of the canister to let the spring apply the brakes. However, in normal driving the spring part remains pressurized when braking. Air is allowed into the other part of the canister which pushes against a diaphram and applies the brakes.

If you should ever run into one of these canisters and are curious about how they work, by all means take one apart BUT ONLY IF YOU WISH TO DIE! The spring in there will cause the canister to come apart like a cannon upon disassembly.

EDIT: Sorry Ivan, my post was in response to the one previous to yours.